Studying mechanisms underlying asymmetric histone inheritance during asymmetric division of Drosophila male germ line stem cell Project Summary: A broad definition for epigenetic phenomenon refers to effects on gene expression or function that are inheritable through cell divisions without altering the primary DNA sequences. Epigenetic mechanisms play important roles in regulating stem cell identity and activity. Failure in appropriate epigenetic regulation leads to abnormalities in stem cell behavior, which underlies many diseases such as diabetes, muscular dystrophy, neurodegenerative disease, infertility, and many forms of cancer. However, a long-standing question has been whether stem cells retain their epigenetic memory during divisions. And if so, could abolishment of stem cell- specific epigenetic memory lead to diseases? Many types of stem cells undergo asymmetric cell divisions to give rise to two daughter cells with distinct cell fates: a self-renewed stem cel and to another daughter cell that differentiates. Recently we found that during the asymmetric division of Drosophila male germ line stem cell (GSC), the preexisting histone 3 (H3) are selectively segregated to the GSC whereas newly synthesized H3 are enriched in the differentiating daughter cell. Our studies provide the first direct evidence that stem cells retain preexisting histones during asymmetric cell divisions in vivo, which may contribute to maintain their unique epigenetic memory. We hypothesize a two-step model for asymmetric H3 segregation: during S phase preexisting histones and newly synthesized histones are differentially incorporated to two sets of sister chromatids; then, during mitosis the set of siste chromatids incorporating preexisting histones are segregated to GSCs, while the other set of sister chromatids enriched with newly synthesized histones are partitioned to the daughter cell which undergoes differentiation. Here we propose to use molecular genetics, cell biology and biophysical approaches to understand mechanisms and biological significance underlying both steps, which will have far-reaching impact on understanding epigenetic inheritance in stem cell maintenance and differentiation.